Training
There are many ways to train for a walking marathon or half marathon. You should select an approach that is appropriate for you and your situation.
If you are strongly self motivated, and want to train on your own, the book The Complete Guide to Marathon Walking by Dave McGovern would be a good choice. It is an excellent reference for long distance walking, and covers both marathon and half marathon distances. Available at amazon.com, or directly from Dave McGovern
If you want to be led through a training program by a professional, activetrainer.com has marathon and half marathon training programs for walkers and for nordic walkers. The Walk Program also incorporates cross-training and rest days to allow your body to acclimate and adapt to the progression of the program. Week by week, your body will adapt, get stronger and walk longer. In addition to the daily training schedule, this program contains weekly inspirational messages from best-selling author and Runner's World columnist John "the Penguin" Bingham. His words will help you stay motivated so that you can achieve your goals! These programs are provided as an extra cost option that you can elect during registration.
Some athletic clothing specialty stores (for instance The Running Room) have structured training programs for walking a marathon or half marathon. Check with the stores in your area.
Many people find that they train more regularly with a group. The peer pressure from a regular group walk is impressive. Organize your own training group!
Whatever choice you make, get out there and walk!